Indicator circuits for electric fuse devices

ABSTRACT

The first and the second terminals of a main fuse device are respectively connected with the first and the second inlet of an electrical element or circuit which is normally in a first state as long as the voltage across the said terminals is negligible, but is brought to a second state by the voltage which appears across same when the main fuse device has blown, a voltage reducing resistor being inserted between said first terminal and said first inlet. In order to protect this resistor an auxiliary fuse device is disposed between same and said first terminal, while the point of junction between this auxiliary fuse and the said resistor is connected with the second terminal of the main fuse device by a threshold element such as a neon bulb. When the main fuse device blows, the voltage across the neon bulb reaches the threshold thereof and a heavy current flows through the said bulb and through the auxiliary fuse device which blows in turn. The indicator circuit may be provided with feed back means to maintain it in its second state after the blowing of the auxiliary fuse device.

Waited tates Luquain atet I INDICATOR CIRCUITS FOR ELECTRIC FUSE DEVICES [75] Inventor: Jean Claude Luquain, Lyon, France [73] Assignee: Societe Lucien Ferraz & Cie, Lyon,

France [22] Filed: June 20, 1972 [21] Appl. No.: 264,596

[56] References Cited UNITED STATES PATENTS Crockettuu ..317 40 A lmmel et al., ..340/25O X Primary Examiner-James D. Trammel] AttorneyArthur E. Dowell, Jr'. et a].

[ 11 3,729,656 [451 Apr. 24,1973

[ 5 7 ABSTRACT The first and the second terminals of a main fuse device are respectively connected with the first and the second inlet of an electrical element or circuit which is normally in a first state as long as the voltage across the said terminals is negligible, but is brought to a second state by the voltage which appears across same when the main fuse device has blown, a voltage reducing resistor being inserted between said first terminal and said first inlet. In order to protect this resistor an auxiliary fuse device is disposed between same and said first terminal, while the point of junction between this auxiliary fuse and the said resistor is connected with the second terminal of the main fuse device by a threshold element such as a neon bulb. When the main fuse device blows, the voltage across I the neon bulb reaches the threshold thereof and a heavy current flows through the said bulb and through the auxiliary fuse device which blows in turn. The indicator circuit may be provided with feed back means to maintain it in its second state after the blowing of the auxiliary fuse device.

4 Claims, 1 Drawing Figure Patentd April 24, 1973 INDICATOR CIRCUITS FOR ELECTRIC FUSE DEVICES This invention relates to indicator circuits for electric fuse devices.

It is conventional in the art to associate to a fuse device, such as a fuse cartridge, an indicator adapted to indicate that the fuse has blownfThese indicators may be either mechanical or electrical. Electrical indicators generally rely on the voltage which appears across the fuse device when the fusible elements thereof have been destroyed. They may comprise for this purpose an electrical element or circuit, such as a thyristor or a flip-flop, which is in a first state when the said voltage is'negligible, i.e., under normal conditions, and which is brought to its second state when the said voltage becomes substantially equal to the voltage of the line in which the fuse device is mounted, i.e., when the said device has blown.

Since the normal operating voltage of the elements which form an electrical indicator is generally much lower than the line voltage, there must be provided some voltage reducing means, as for instance a resistor of relatively high value. Such a resistor may be submitted to quite high electrical stresses owing to the surge which appears when the fuse blows. It has therefore to be very carefully insulated and its dimensions should be large enough to avoid any overheating and arcing. It is to be noted in this respect that if an arc is formed across such a resistor, the full current ofthe line passes through it; in other words the fuse device is rendered ineffective and the whole indicator and adjacent parts may be destroyed by fire.

It is an object of the present invention to avoid these drawbacks of electrical indicators.

In accordance with the invention an auxiliary fus device is inserted between the voltage reducing resistor and the first terminal of the main fuse device, while a threshold element is mounted between the second terminal ofthe main fuse device and the junction ofthe resistor andof'the auxiliary fuse device.

The auxiliary fuse device may be formed of a fuse device 2 have blown, this difference may rise to several hundred volts or more, as the case may be.

A negative wire 6 extends from the first terminal 3, this wire being connected by a Zener diode 7 with a reference point A which is itself connected with the second terminal 4 of the main fuse device 2 through a current limiting protecting resistor 8 and an auxiliary fuse device 9, the latter being preferably formed of a fuse cartridge adapted to operate under the full voltage of line 5, but with a quite low intensity, as for instance some milliamperes. The point ofjunction B between resistor 8 and the auxiliary fuse device or catridge 9 .is connected to the negative wire 6 through a neon bulb 11.

The indicator circuit proper comprises a first transistor T1 of the NPN type which has its base connected to point A through a wire 12 and a resistor 13. Wire 12 is also connected to the negative wire 6 through a polarizing resistor 14, a filtering capacitor 15 being mounted between wires 12 and 6 in parallel with the said resistor 14.

The emitter of transistor T1 is connected to the negative wire 6 while its collector is connected to the base of a second transistor T2 through a wire 16 and a resistor l7. Transistor T2 is of the PNP type. lts emitter is connected through a wire 18 with the positive terminal of a direct current source 19 the negative terminal of which is connected to the negative wire 6. The collector of transistor T2 is connected to an indicating lamp 20, the latter being itself connected to point A through an anti-return diode 21 and to wire 6 through a resistor the presence of resistor 14, of relatively low value, the

cartridge having-a high interrupting capacity. As to the threshold element it should be adapted to conduct thev full current which corresponds to the blowing of such a cartridge, without being destroyed before the said car-' tridge has interrupted itselectric circuit. A neon bulb is particularly convenient in most cases. In any case the voltage applied to the voltage reducing-resistor can never exceed the threshold of the neon bulb or like element.

The annexed drawing shows an embodiment of an indicator according to the present invention. I

Reference numeral 1 designates a direct current electric power line in which there has been inserted a main fuse device 2, such as, forinstance, a-fuse cartridge having a nominal rate substantially equal tothe maximum load carried by line I, the said fuse cartridge having first and second terminals respectively designated by reference numerals 3 and 4. The arrow 6 indicates the direction of the electric current which flows through line 1 and therefore terminal 3 may be described as negative with respect to terminal 4. Under normal operating conditions the voltage difference between terminals 3 and 4 is only a small fraction of a volt, but when the fusible elements of the main fuse potential of the base of transistor T1 is practically equal to the potential of its emitter. Transistor T1 is therefore non-conducting. The base of transistor T2 is thus practically'unpolarized, as if it were actually unconnected. Transistor T2 is therefore also non-conducting and lamp 20 remains'at rest. The Zener diode 7 and the neon bulb 11 are also inoperative. 1 If now the main fuse device 2 blows, the full line 'voltage is applied to bulb l1 and since this voltage exceeds the threshold of the bulb (about volts), the latter becomes conducting and a heavy current flows therefore through the auxiliary fuse device 9 which blows almost immediately. But the voltage which has been applied across bulb 11 has been transmitted to point A through resistor 8 and from point A to the base of transistor T1 through resistor 13. and wire 12. Transistor Tl thus becomes conducting and it polarizes the base of transistor T2 which also becomes conducting. The indicating lamp 20 is therefore illuminated, its circuit being closed to the negative wire 6 by resistor 22. The voltage drop across resistor 22 is applied to the anti-return diode 21 and it is therefore transmitted to point A which thus remains positive with respect to the negative wire 6 after the auxiliary fuse device 9 has blown, thus maintaining transistor T1 in the conducting state. It is to be noted that when point A becomes suddenly positive just after the main fuse 2 has blown, diode 21 prevents any undesirable flow of current from the said point towards the negative wire 6 and thus permits the setting of transistor T1 to the conducting state.

The potential of point B with respect to the negative wire 6 can never exceed the threshold of bulb 11, provided the auxiliary fuse device 9 interrupts the circuit before the said bulb is wholly destroyed. The voltage reducing resistor 8 cannot therefore be submitted to a voltage exceeding the threshold of bulb 11. As to the potential of point A with respect to wire 6, it is limited by the Zener diode 7 in the conventional manner. It is to be noted in this respect that resistor 22 only acts to by-pass a fraction of the current from lamp 20, which would otherwise flow through diode 21 and 7. Resistor 22 might therefore be dispensed with if these diodes are properly dimensioned with respect to the normal current of lamp 20.

If now it is assumed that line 1 is an alternating current line, under normal conditions the quite small AC voltage of point A with respect to wire 6 will be negligible. When the main fuse device 2 has blown, as soon as pointA becomes positive the circuit will operate as above described for the case of DC. Of course this implies that if the main fuse device 2 interrupts the current when terminal 4 is negative with respect to terminal 3, the auxiliary fuse device 9 will not interrupt its own circuit before terminal 4 is positive. Such is the case in actual practice if the auxiliary device 9 is formed of a conventional fuse cartridge.

It is obvious that the auxiliary fuse device 9 could as Well be inserted between terminal 3 and the negative wire 6.

I claim:

11. In connection with a main fuse device having a first and a second terminals, an indicator circuit comprising:

electric means having a first and a second inlets, said means being normally in a first state, but being adapted to be brought to a second state by a predetermined voltage applied across said first and second inlets when said main fuse device has blown;

an electrical indicating device actuated by said electric means when same have been brought to their second state;

a voltage reducing resistor having a first and a second end, with the second end of said resistor being connected to the first inlet of said electric means;

conductor means to respectively connect the first terminal of said main fuse device with the first end of said voltage reducing resistor and the second terminal of said main fuse device with the second inlet of said electric means;

a threshold conducting element mounted between the first end of said voltage reducing resistor and the second inlet of said electric means, said element being normally inoperative, but forming a by-pass circuit across said main fuse device when same has blown to limit the voltage applied to said voltage reducing resistor;

and an auxiliary fuse device inserted on one of said conductor means to break said by-pass circuit before said threshold element is destroyed.

2. In an indicator circuit as claimed in claim 1, said threshold conducting element being formed of a neon bulb.

3. In an indicator circuit as claimed in claim 1 said electrical means further including a source of electric current, said source feedlng said electric indicating device, and feed back means to derive from the current supplied to said indicating device a voltage applied across said first and second inlet to maintain said electrical means to their second state after said auxiliary fuse device has blown.

4. In an indicator circuit as claimed in claim 3, said feed back means including a resistor interposed between said indicating device and said second inlet, and an anti-return diode interposed between said indicating device and said first inlet. 

1. In connection with a main fuse device having a first and a second terminals, an indicator circuit comprising: electric means having a first and a second inlets, said means being normally in a first state, but being adapted to be brought to a second state by a predetermined voltage applied across said first and second inlets when said main fuse device has blown; an electrical indicating device actuated by said electric means when same have been brought to their second state; a voltage reducing resistor having a first and a second end, with the second end of said resistor being connected to the first inlet of said electric means; conductor means to respectively connect the first terminal of said main fuse device with the first end of said voltage reducing resistor and the second terminal of said main fuse device with the second inlet of said electric means; a threshold conducting element mounted between the first end of said voltage reducing resistor and the second inlet of said electric means, said element being normally inoperative, but forming a by-pass circuit across said main fuse device when same has blown to limit the voltage applied to said voltage reducing resistor; and an auxiliary fuse device inserted on one of said conductor means to break said by-pass circuit before said threshold element is destroyed.
 2. In an indicator circuit as claimed in claim 1, said threshold conducting element being formed of a neon bulb.
 3. In an indicator circuit as claimed in claim 1 said electrical means further including a source of electric current, said source feeding said electric indicating device, and feed back means to derive from the current supplied to said indicating device a voltage applied across said first and second inlet to maintain said electrical means to their second state after said auxiliary fuse device has blown.
 4. In an indicator circuit as claimed in claim 3, said feed back means including a resistor interposed between said indicating device and said second inlet, and an anti-return diode interposed between said indicating device and said first inlet. 